The present invention relates to a method of inkjet printing of inks containing nanoparticles, and to printing apparatus for carrying out the method.
The method and apparatus are particularly, but not exclusively, suitable for printing of functional inks for electronic applications, where a high density of interconnecting particles and small feature size of the printed pattern are required.
Printing of functional inks has a long tradition in the electronic field. For example, pigment based inks are used to screen print interconnections and resistors on printed circuit boards. In these applications the thick film inks used consist of a vehicle, and pigments of silver and carbon respectively, where the pigment particles can have a dimension in the nanometer range. More recent developments are aimed at printing not only the passive components of a circuit, but also active components. One example is the disclosure of printed nanoparticulate silicon in International patent application WO 2004/068536 of the present applicant, providing semiconducting layers in devices like solar cells and transistors.
Traditionally most functional materials have been printed by conventional printing techniques, such as screen printing and flexography, both of which require the fabrication of a master pattern (e.g. a screen or printing plate) for each design to be printed. It is generally held desirable that digital printing methods, such as inkjet printing, should be applied, because of their flexibility in use and higher spatial accuracy. However, to prevent clogging of the ink jet nozzles, ink jet printing requires relatively dispersed solutions of particles and a low viscosity ink. This makes this method unsuitable for certain applications in the electronic field, in which a high density of particles has to be brought to a specific position on a substrate to achieve the required functionality of the printed pattern.
With regard to the deposition of small feature size patterns, inkjet printing of solutions containing nanoparticles, which provide functional properties to a printed structure, is known. The most common applications are ink jet printing of conductive traces for circuits, using conductive nanoparticles, e.g. silver nanoparticles, dispersed in the ink. In such applications a low resistance is obtained by heat treatment, with the effect of removing the dispersant, and subsequent sintering of the nanoparticles. A more recent development in functional layer deposition is inkjet printing of nanoparticulate transparent conducting oxide, where the patterned structure and the particle packing is controlled by a treatment with electromagnetic radiation in the drying process.
Another method to enhance the precision of patterning in ink jet printed structures composed of functional inks, including inks containing nanoparticles, is electrohydrodynamic jet printing, described by Jang-Ung Park et al (Nature, Vol 6 (2007) p. 782). In this case the resolution of the printed pattern is enhanced by an electrostatic field, applied to a microcapillary nozzle of the ink jet equipment, which shapes and controls the motion of the drops ejected from the nozzle. However, electrohydrodynamic jet printing has no effect on the density or arrangement of particles in the printed structure, and post processing is necessary to achieve the desired properties.
In certain applications the functionality of a deposited layer, containing particles in general, and nanoparticles in particular, is provided by an interconnecting network of these particles. To achieve compaction of such layers a modification of electrophoretic deposition has been disclosed by Tuck in GB2355338 for field emitting displays. This work teaches the forced sedimentation of particles from a dilute solution of the binder material by an applied electric field. The amount of binder in the solution is carefully calculated so that, after evaporation of the solvent, the sediment is held in place at the bottom of a microscopic well. As in other conventional electrophoretic deposition techniques, used for coating from a bath of solution, there is no forming of the pattern or control of the fluid flow during the process.